I was five years old when my single mother was attacked by a ghost in her bed in the middle of the night. She awoke suddenly under the pressure of two unseen hands pushing her down flat against the bed and holding her there. For several minutes she struggled, unable to speak or move. Finally she broke free and scrambled out of the room, and spent the rest of the night on the floor of the room that my brother and I shared. She never went back into her own bedroom alone again. And so I grew up with this history, hearing ghost stories from time to time that other people told, but knowing that we had actually had a real ghost in our home when I was young.

I was an inveterate reader of books about monsters and ghosts — everything from Bigfoot to Dracula, from banshees to fairies, from zombies to werewolves — and one subject that particularly piqued my interest was that of nocturnal assaults. Noctural assaults are attacks just like that suffered by my mother, though often more graphic: the attacker can sometimes be a visible apparition. I was highly intrigued to learn that the physical descriptions of the attackers have been eerily similar over the ages, varying by country and sometimes by century. In Anglo cultures the most common attacker is called the Old Hag, a terrifying old woman dressed in black rags who holds her victims down in their beds or even sits on their chests with her full weight. References to the Old Hag and her nocturnal attacks go back as far as the Middle Ages. She's been part of our history for so long that if you haven't slept well, you're said to look "haggard". In India she is the Mohini, a beautiful but deadly enchantress. As often as the Anglo attacker is decribed to look like an old hag, attackers in India are just as frequently described as a beautiful young woman with terrible powers. In Slavic cultures, the most frequent description is of an elf-like gypsy man with wild glowing eyes who sits on your chest, riding you like a horse. The more I researched it, the more cultural groups I found to have their own unique noctural assault perpetrators.

As a budding young scholar of the supernatural, I was fascinated by these cultural commonalities. Similar attacks, throughout history, made by specific attackers who stayed within their own cultural communities. But then in the late 1960s, things changed. A new attacker began muscling in on the Old Hag's territory, and quickly took over responsibility for most of the attacks reported in the United States. Do you know who I'm referring to yet? In 1965, Betty and Barney Hill went public with an episode they said happened to them in 1961, when they were abducted from their car by aliens, and suffered terrible medical experiments aboard a spacecraft. Curiously, the attack they described bore no resemblance to a classical nocturnal assault; however the creature they described — an alien of the type we commonly call a "gray" — became America's new supernatural superstar. Nocturnal assaults continued to happen at the same frequency that they always had, but now the reported attacker was, more often than not, a gray alien. The gray alien burst upon the scene of America's consciousness just as the Old Hag was beginning to seem a little outdated and, well, haggard. Just as children in India grew up with stories of the Mohini as the evil specter who might paralyze you in the middle of the night, we're now in a generation of Americans who have heard that gray aliens are those little beings who are going to come into your bedroom at night and attack you.

Is it really as simple as that? Is the attacker that your scared brain visualizes based solely on what your cultural experience tells you to expect?

It was about 25 years after my mom's attack that I first heard of sleep paralysis, which, as you probably know, is the clinical name of these nocturnal assaults. Sleep paralysis can be characterized by an inability to speak or move, a feeling of intense crushing weight on the chest, and/or hallucinations which can be visual, auditory, tactile, and even include strange smells. It happens only during REM sleep, often just as it's beginning or ending. Sleep paralysis is five times more likely to happen to people sleeping on their backs, facing up. Drugs such as Prozac have been found effective in controlling sleep paralysis attacks. Although most sleep paralysis episodes do not include the visual apparition, more than enough do include it to account for all reported nocturnal assaults. Sleep paralysis is well understood, well documented, and is an accepted psychological phenomenon in the medical professional.

So why, then, did it take me a further several years before I made the connection between my mom's attack and sleep paralysis? I had spent so many years fully believing that my mom had been attacked by a ghost that it never even occurred to me to seek more reasonable explanations elsewhere, even when the obvious answer was staring me in the face, literally, as I was reading books about it. Perhaps this is the same reason that even in an age where most people have at least heard of sleep paralysis, believers in alien abductions and noctural ghost attacks firmly stick to paranormal explanations for their own sleep paralysis experiences.

Many believers, when confronted with this explanation for their experience, will point out differences between their experience and the known symptoms of sleep paralysis. Of course, visual, audible, and tactile hallucinations are part of the known symptoms of sleep paralysis, so it's kind of hard for them to come up with details that can't be attributed to known sleep paralysis effects. And that's an awkward position for the scientific profession to be in: no matter what the believer reports, we can explain it with "It's a hallucination." That makes us sound exactly like Young Earth Creationists explaining everything with "God did it," no further evidence needed. The difference is that we can actually test sleep paralysis sufferers, and whenever we do, we end up with video of them lying in their bed looking paralyzed, with a conspicuous absence of gray aliens, Mohinis, or Old Hags in the room.

So it took over thirty years, but I finally did explain my mom's noctural assault, at least to my own satisfaction. You might wonder what her own assessment is, in light of this explanation. She went to medical school, spent her whole career in biotech, has a very scientific mind, and is convinced to this day that she was attacked by a ghost. She never read the Betty and Barney Hill story.